Bifocal-lens blank



Aug. 19 .1924. j Re. 15,897 G. A. CLEMENT IFocAL Lans, BLANK Original Filed April 9. 1923 .as .so .75 |.oo `1.25 Lso L15' 2.00 2.25 25o 2.15 5.00 3.15 35o 1575 4.00 4.50'

El er cr cr er er er cf cr cr cr cr cr r A TTORNEY` Reissued Aug.` 19, 1924.

UNITED STAT s Re. 15,897 PATENT OFFICE.

GEORGE A. CLEMENT, OF MOUNT VERNON, NEW YORK, ASSIGNOR GENERAL OPTI- CAL COMPANY, A CORPORATION F NEW YORK.

\ `BIFOCAL-LENS BLANK.

Original No. 1,494,889, dated May 20, 1924, Serial No. 630,880, filed April 9, 1923. Applitmtion for reissue led June 27, 1924. Serial No. 722,876.

T o all whom it may concem:

Be it known that I, GEORGE A. CLEMENT, a citizen of. the United States, residing at 'Mount Vernon, in the county of VVestchestel1 and State of New York, have invented certain new and useful Improvements in Bifocal-Lens Blanks, ofwhich the following is a full, clear, and exact description.

In the manufacture of blanks from which lo fused bifocal lenses are produced, the practice is to grind and polish a recess or de-` pression in one face of a major portion or piece of glass near one edge thereof, and seat therein 'a smaller piece of glass, known 16 in the trade as a button, having itsunder surface properly shaped and prepared to unite with that of the recess and havin an index of refraction greater than that o the major piece. In meniscus lenses the recess i0 or depression is in the outer orconvex face.

The two pieces of glass are then subjected to heat until they unite by fusion forming one integral lens blank, and the finishedlens is madeby grinding and polishing the two faces of the blank to the proper curvatures according` to the desired lentlcular and cylindrical powers. While the curvature of the bifocal side is uniform throughout its entire area, the focal distance through the 80V segment or area occupied by the button, which constitutes the reading segment or field', is shorter than that through the rest of the lens which constitutes the distance field, because of the difference in refrac- 86 tive index through the two fields. This result is obtained, for example, by makingl the major iportion of crown glass and the button of int glass.

Since the power of a lens depends upon 40 the curvatureI of its faces audits index of refraction, it follows that the added power through the reading field depends both upon the curvature of the recess or depression, i. e., the curvature on the inner face of 65 the button, and the curvature of the outer or bfocal face of the major lenswhich curvature .is also common to the reading segment. According to present standard prac,

tice the curvature of the depression varies U in a complete stock of fused bifocal lens blanks from a maximum concavity for the highest addition to a plane or substantially glane surface, the refracthe index of the int glass buttons being constant through out the entire range. In rare cases where the added power called for b a prescription is very high the manu acturer may make a special pair of blanks using a flint glass of higher lndex of refraction 1n order to reduce the concavity and hence the thickness of the lenses, but this is only resorted to in special cases and requires the services of a specialist to determine the curves which it is necessary to grind to .get the desired addedv powers.

In the standard practice the meniscus or torio blanks are put out in a number of socalled base curves in order to minimize the work of the lens grinder in reducing the surfaces of the blanks? to the desired curvatures. The outer or convex face of the blank may be finished to any curvature Within the range of the tools of the lens grinder, and the lens grinder will select a blank having a base curve nearest to that of the finished surface. It is manifest, however, that with a given blank the added power through the reading field will depend upon the final curve, of the convex face, since this determines the loutside curve of the minor lens. Therefore in order to obtain a desired addition, as this added power is commonly called, the inside curve of the llint glass button must vary with the curve on the outer face. This makes necessary a great number of curvatures for the recess or inside face of the button for each base curve, in order that the dispenser may be able to fill any ordinary prescription, and requires him to carry a large number of blanks in stock agl makes his initial investment considera e.

My invention very largely reduces the num er of blanks in a complete set capable of satisfying the ordinary prescri tions. Briefly stated, it contemplates a set o fused bifocal lens blanks in which the geometrical surface of the recess is constant. `This has many advantages, some of which will presently be pointed out, and I have determined by calculation and test that the refractive index of the iiint glass used for the buttons may be so varied as to provide the requisite range of additions without modifying the inside curve ofthe buttons, and at the same time permitting the grinder to use the pres-V ent standard surfacin tools.

By way of illustration I shall explain my j operated on at one time. And in any event invention as it will be employed so that the two surfaces which are. united by fusing are plano. The advantages in fusin 4two plano surfaces together as compare with the fusing oftwo spherical surfaces 'will be ,a preciated'by those skilled in theV art. In t efpresent practice in which the contacti sur ace-of the button is convex, i-t is groun to a eater curve than is the recess and is seater in the recess and held in contact at one point at the initiation of the fuslin operation. The button is` then heated-*untll it gradually drops down from 'the point of contact outwardly and coalesces with the wall of `the depression', this method bein employed to exclude all air andy evolve ses from between the two surfaces. vvFor t is (purpose the fusing temperature re quire is so great that -the glass at the surfaces is caused to deteriorate and the contact surfaces are freuently distorted. This is entirely eliminate in the fusing of two plano surfaces together since a oonsiderably lower ltemperature is required therefor and it isunnecessary to heat the lass to such .a degree asV to effect it deleteriously.

, Another decided advantage in a set vof blanks in which the `countersmk or recess is.

practically eliminated, irrespective of the addition, is the fact that the nished lenses may be thinner andcorrespondingly lighter, since they enable the lens grinder to grind the concave face of lenses of hi h addition as far down as that of lenses o the lowest addition.

lAnother feature of my invention is the fact that it reduces the chromatic aberration of thel reading field for the higher additions as compared with the present type in which flint' glass of the same index `of refraction and with its relatively hi h dispersion is used for bothV the high ,and ow additions;

yStill another advantage of the set of blanks comprehendedV by my invention is the fact that the cost of manufacture is materially reduced. For example with the' plano interfaces vthe crownrdiscs may be much more expeditiously prepared to receive the buttons than is-the case wherethc countersinks are spherical; they 4 are more.

easilyblocked lup and many more maybe it is evident that a considerable savin' in by time and grinding apparatus is eifeete having all of the interfaces the same geov metrical surface, and that economy-results from the saving of time and heat required for the fusing operation, and from the substantial reduction in the number of defective blanks.-

Many other advantages in m invention than those above pointed out wi l appear to those skilled in the art. T shall now explain one way in which my invention may be reduced to practicer and will thereafter point out my invention in claims.

\ bifoca'l side has been surfaced down to the desired curvature, as indicated by the dotted line in Fig 2;

Fig. 4 is a table of seven different flint glasses which may be -employed in carrying out theu invention,-giving their indices of refraction on the D line together with the surface curvatures of lenses made from them -which will' be necessary to effect an addition of one diopter;

Fig. 5 isa chart illustrative of the use of the Seven flint glasses in conjunction with a major lens of crown glass of refractive index 1.523, and showing the numbers of the flint glasses and the surface curvatures which will provide the customary additions; and

Fig. 6 isv a chart similarto that of Fig. 5 except that it is limited to three different intglasses-of the seven tabulated in Fig. 4. l 'Figs 1, 2 and 3 show a rough major blank 1 of crown glass and a button 2 of fiint glass. A small portion 3 of the convex face of the major blank has been ground and polished off to a plane surface.v The button 2 is a plane' disc, the under face of which is ground and polished to a. plane surface. Preferably the twoparts willbe placed in-ja furnace in the relation illustrated in Fig. 1, with a small insert or Wedge 4 underneath vthe lower edge o f the button so as to hold it in tilted relation with its upper edge resting at oneV int upon the surface of the crown glass lank. Under the influence of the heat the Hint glass but-ton becomes plastic and drops down so as yto conform to and coalesce with the Vsurface ofthe crown glass,as shown in Fig. 2. I'nproducing the finished lens from this fused blank,

the top surface is ground off to the desired curvatureso as, to includei a suicient portion ofthe Hint glass button, for example as illustrated by the dotted line in Fig. 2. The underface of thebl'ank is then ground according to prescription, and the finished lensl is cut out thedesired shape, for in stance as' indicated in the dotted outline in Fig. 3. Itais evident that the added power through the reading segment depends u on the index of refraction on the flint, g ass and upon the curvature of the outer face of the lens, since the lower face of thel button is plano.`

I have'determined that if the seven flint glasses tabulated in Fig. 4` are ground lan'oconvex with the convex curvatu'ies in icated in the third column of ig. 4, they will all loc add one diopter in lenticular power when used in conjunction with a maJor blank crown glass having an index of refraction of 1.523. In other words, if a blank is made similar to that'illustrated in Figs. 1, 2 and 3 with the button 2 made of #l flint glass with an index of refraction on the I) line of 1.5665, the addition throu h the reading field will be one diopter if t e convex face is ground to a curvature offtwelve diopters. From this it follows that if the convex face of the blank be round to six diopters, the added power wil be .5() diopters, and other additions may be similarly computed.

From the table of Fig. 4 lit follows that if the button 2 be made of #2 flint glass having an index of refraction of 1,5753, the addition will be one diopter if the curvjature of the outer face of the blank be ten diopters. If the button be made of #3 flint glass havin an index of refraction 1.5884, the addition will be one diopter if the outer face be ground to eight diopteis. If the button be #4 flint glass with an index of refraction of 1.6102, the addition will be one diopter if the curvature on the outer face be six 'diopters If the button be made of #5 flint glass havin an index of refraction of 1.6537, the ad 1tion will be one diopter if the curvature of the outerA face of the blank be four diopters. If the button be made of #6 flint glass having an index of refraction 1.6973, the addition will be one diopter if the curvature of theI outer face be three diopters. If the button be composed of #7 flint glass having an index o refraction 1.7845, the addition will be one diopter if the curvature of the outer face of the blank be two diopters.

In the chart of Fig. 5 there is indicated the curvature. of the outer face and the number of flint glass which would serve to roduce additions Ifrom .25 to 4.50. It wil be understood that all the .possible combinations are not therein illustrated, but only such as might easily and reasonably be employed In the first column is shown one combination which will produce an addition of .25 diopters. This is #1 flint glass with a curvature of three diopters, F standing for flint `glass and C standing for curvature. If #1 flint glass adds one diopter with the curvature on the outer face of thev blank of twelve diopters, it therefore follows that it will add .25 diopters if the outer curvature is three diopters. In the second column there are shown .four combinations of flint glass and surface curvatures which will produce an addition of .50 diopters. These are the first four of the seven flint glasses with the curvatures just half of that indicated in the third column of Fig. 4. In the third column ofvFig. 5 there are shown four combinations which will produce an addition of 75, these being the first four numbers of until the highest addition flint glass; with the curvatures three `quarters of the respective numbers appearing f1n the third column of'Fig. 4.; 'and so itwill beA seen that the successive columns are sim ly the result of mathematical computation 4.50 dioptcrs, is reached. It will be seen that the cul/'vature of #7 flint glass'in order to produce this addition will. need to have 41/2 times the curvature which is set opposite it in Fig. 4; i. e., a curvature of nine diopters. Within this range of additions are included all of the ordinary prescriptions.

These blanks could be made on any base curve plan and are readily adaptable to the present three base curve plan, viz.,` three diopters, six diopters and nine diopters. Th1s would give a maximum of twenty one numbers of blanks to satisfy all prescriptions, i. e., seven different numbers of flint As will' glass of each of the base curves. readily be seen from an inspection of the chart of Fig. 5, all of these numbers would not be necessary.

The selectin of the proper blank for each addition will e a simple matter. For the .25 diopter addition, the blank selected would be that having #l Hint glass for the button and a base curve of three diopters. For' the .50 diopter addition the 'grinder would select either a blank having a #1 flint `glass and a six base curve, or one having a #2.flint glass on a three base curve, or a #3 on a three base curve, or a #4 on a three base curve according to which one of the four possibilities indicated in the second column of Fig. 5 he concluded'to employ. While he might select a six base curve blank for the second possibility, it is lpreferable to grind the surface two diopters stronger than the base curve rather than one dio ter weaker. So with each. addition the ens grinder by lsimply referring to this chart. can determine which blank to use. For example in the column lfor 2.75 diopter addition or the first possibility he would use a blank having #5 int glass and a nine base curve. For the second possibility he would use a #6 Hint glass von a nine base curve. For the third possibility he would use a #7 flint glass on a. six base curve. In every instance the base curve selected would depend upon the curvature to be round on the rear face in order to give the esired power through the 'major lens.

It will readily be appreciated that for the corrections which are ordinarily met with it is not necessary to use as many as the seven different indices of flint glass. In only exceptional cases do prescriptions call for additions below .50 diopter or above 4.00 diopters. For instance, as appears from the chart of Fig. 6, prescriptions within that range can be taken care of by a set of blanks the buttons of which are made from only numbers 4, and 6 of the Hint glass tabulated in Fig. 4. It will be` observed that these -combinations could be satisfied with only seven numbers of blanks, as fol- 5 lows: a #4 flint glass on a three base curve;

a#4 Hint glass on a six base curve; a :,lt

the indices vof refraction of both the Hintand crown glasses as above mentioned. mayV be modified without departing from the scope of my invention as pointed out inthe appended claims. 1

claim: y

1. A roduct of manufacture comprising a set of ifocal lens blanksformed by fusing a small piece of glass having a relatively y high index of refraction to a'niajor piece of lass of lower index of refraction, the set eing` made up of a plurality of series ac- 3 cordlng to base curve and each series being made up of a plurality of units according to the index of refraction of the small piece of lass, the index of the small piece of glass ing different for each unit of a series but being the same for the corresponding units of the several series and the base curve be-y ing different for the several series but being the same for the units of each series.

2. 'A product of manufacture comprising series.

msec

a set of bifocal lens blanks formed by fusing 40 a small piece of glass having a relatively high index of refraction to a. major piece of glass of lower index of refraction, the blanks all having substantially the same` geometrical surface between the two pieces of glass and the set being made up of a plurality of series according to base curve4 and each series being made up of a plurality of units 'according `to the' index of refraction ofthe small piece of glass, the index of the small piece of glass `being different for each unit of a seriesl but being the same for the corresponding units of the several series and the base curve being different for the several series but being the same for the units of each series. y t

3. A product of manufacture comprising a set of bifocal lens-blanks formedby fusing Y a small piece of glass having a yrelatively high index of refraction to a major piece of glass of lower index of refraction, the blanksfall having substantiall' a plane surface betweenthe two pieces o lass and the set being made up of a plura ity of series according to base curve and each series be- 'ing made up of a plurality of units according to the index of refraction of the small piece of glass, the index of the small piece of glass being 'different for each unit of a series but being the same for the corresponding units of the several series and the b ase curve being different for the several series but being the same for the units of each In witness whereof, I hereunto subscribe my'signature.

A'GEORGE A'. CLEMENT. 

